And we begin this hour in Berlin. This evening, a truck slammed into a crowded Christmas market there. It happened in the western part of the city in the heart of a shopping district.

ARI SHAPIRO, HOST:

According to police, at least 12 people have been killed, and dozens more have been injured. Police say they are investigating the incident as a possible terrorist attack. We reached one witness in Berlin, Oliver Schneider. He's 19, originally from New Zealand and moved to the city a few months ago. He was shopping and saw the immediate aftermath of the incident.

OLIVER SCHNEIDER: We immediately had about 30 people running towards us, looking really panicked. Obviously our first thought was they're running for a bus or something. But there was sort of a look in their eyes which nobody really has running for a bus. And that was when we realized they were running away from something. So we walked up a little bit, and we saw just this massive black truck halfway in the road, halfway in the market.

SHAPIRO: That was one witness's account. We're going to get more on this story now from NPR's Soraya Sarhaddi Nelson in Berlin. And Soraya, what can you tell us about the driver of this truck?

SORAYA SARHADDI NELSON, BYLINE: We don't know a whole lot about him. We know that there is a person in custody, according to Berlin police. But they say that they are not sure that it is the driver. But he was driving a black truck with Polish plates. It tore down one street and then into the square where this Christmas market was at. And there were no break marks according to investigators.

There was another person in the truck who they're referring to as a passenger - the police referring to as a passenger. And he died on the scene. According to reports from Poland, this truck belonged to a Polish haulage company registered in Gdansk, and the owner who spoke to Polish Radio said his cousin had actually been driving to Berlin in this truck.

SHAPIRO: What is the situation at the scene now?

NELSON: Well, you have to picture. This is a place that's really crowded at this time of year. It's in a major shopping district of Berlin, and it's been completely cleared of normal shoppers or regular people who would be attending the Christmas market. The only people there now are investigators, rescue workers and journalists.

The Berlin mayor, Michael Muller, says that the situation is under control. Justice Minister Heiko Maas says that the investigation has been taken over by the federal prosecutor general. And everyone in the officialdom, if you will, is cautioning that they're still not a hundred percent sure that this was actually an attack, that it might have just been a runaway truck, like an accident.

SHAPIRO: As we've been saying, this was at a Christmas market. Give us a sense of how popular these are in Germany.

NELSON: Well, they're very popular. They start about four weeks before Christmas. You have to picture little huts with all sorts of handicrafts. And they sell all kinds of foods and of course the very famous Gluhwein, or mulled wine, which is the most popular drink that you would get there.

And this is a place where Germans like to go after work. You know, while they're shopping or after they're shopping in the stores, it's just a fun place for families and for friends to get together. And they're very crowded, especially in this week leading up to Christmas.

SHAPIRO: ISIS had been threatening to increase its attacks on Western European targets. Were German authorities expecting this, and what precautions have they taken?

NELSON: Well, they certainly had - were aware of the warnings, and they had been taking extra steps. I mean I noticed certainly at the Christmas markets, you saw a lot more in terms of police officers there. All the bags were checked.

You have to remember; they had the same sort of fear last year. And I didn't actually notice the level of fear in people going to the markets last year - I mean this year as I did last year. People were very relaxed about it certainly when I was there on Saturday and Sunday night. No one was worried, and even the police looked very relaxed.

SHAPIRO: What are police telling people to do right now?

NELSON: They're telling them to stay indoors - they - that the situation is still clearly not as much under control (laughter) as the Berlin mayor would have you think. They're also very concerned. I mean they're expecting the death toll to rise. That's something that we reporters have been told. And they say that they still need the roads cleared for rescue vehicles.